The present invention concerns an alkaline primary cell in which the negative active material is in the form of powdered zinc and more particularly a primary cell in which the negative electrode is arranged in contact with the casing of the cell and comprising a visual indicator of the state of charge.
It is often desirable for a user to know the state of charge of a primary cell, that is, the capacity remaining available in that primary cell at a determined moment. This is the case when the exhaustion of the primary batteries must be foreseen in advance or, even, when it is sought to detect the causes of a breakdown of a device fed by primary batteries.
U.S. Pat. No. 2,980,754 proposes, for that purpose, to provide a primary cell with an insulating layer, on the outside of the negative electrode, which changes appearance when it is reached by the leaking electrolyte at the end of the charge. That arrangement has the disadvantage of warning the user too late, since the electrolyte has already begun to leak.
Moreover, through French patent No. 71 48 012 of Dec. 21, 1971, published under No. 2,184,475, an air depolarization electric primary cell comprising an outside casing in which at least one transparent zone makes the discharge front of the negative electrode visible from outside, the negative active material being zinc powder in suspension in a gel, is known. Such observation is made possible by the fact that the active constituents of the cell are stacked orthogonally to the lateral wall of the envelope and hence that the discharge progresses substantially in a plane perpendicular to that wall. In a cylinder cell (cylinders, of course, include prisms which are cylinders with a polygonal directrix, as well as cylinders in the usual sense of the word, having a circular directrix) in which the active components are in the form of cylinders fitted into one another, even if the negative electrode has a peripheral position and the positive electrode has a central position, it is not possible to follow the discharge front according to the method used in the preceding patent, since the discharge front is represented by a cylindrical surface parallel to the internal surface of the negative electrode and is therefore invisible through the walls.
An aim of the present invention is to make it possible to observe the state of charge in such primary cells.
The present invention, therefore, has among its objects and features an electric primary cell comprising a negative electrode whose active material basically contains powdered zinc and which is situated in contact with the casing, at least in the vicinity of a transparent portion of the latter, an alkaline electrolyte and a positive electrode, the outside casing of the cell has at least one transparent portion through which a discharge indicator is visible from the outside; the latter being constituted by a perforated sheet made of a material chosen from the group comprising copper and its colored alloys, visible through the transparent portion and in contact with the negative active material impregnated with electrolyte. It is to be understood that perforated sheet means a wire gauze or an expanded metal sheet as well as a solid sheet perforated subsequently. As will be seen further on, when the color of the discharge indicator changes from grey to red or to yellow, this means that the discharge front of the active material has reached its level: consequently, the position of the copper sheet in the active negative material is chosen as a function of the position of the discharge front, that is, of the state of discharge which is to be observed. Such an arrangement also makes it possible to be warned of the approach of the end of discharge with a safety margin adapted to the use of the cell.